Various methods and a circuit arrangements of the above mentioned general type are conventionally known. Such methods and circuit arrangements are utilized, for example, in the field of mobile reception of radio signals, such as in modern automobile radios, whereby various specific techniques are used, in order to ensure an uninterrupted and interference-free reception of the radio signals even under continually changing reception conditions.
A first example of the above mentioned techniques is given by the Radio Data System (RDS), which provides for the transmission of information indicating an alternative frequency on which the same radio program can be received if there is interference on the primary frequency. Based on this information indicating one or more alternative frequencies, the receiver can monitor and evaluate the reception quality available on the various alternative frequencies, and then select the best frequency, i.e. the frequency with the best reception quality, for carrying out the further signal reception. In that context, it is advantageous to provide and operate not only a first receiver (the audio receiver) but also an additional second receiver (a background receiver) that runs in the background in order to monitor and test or evaluate the reception quality of the available alternative frequencies on an ongoing basis. If such a background receiver indicates an alternative frequency having a better reception quality than the frequency presently being used by the audio receiver, then the audio receiver is switched over to this better alternative frequency. As a further possibility, the respective roles of the audio receiver and the background receiver are switched, namely the receiver previously operating as the background receiver will now operate as the audio receiver on the alternative frequency having the better reception quality, while the previous audio receiver will then operate as the background receiver on the other frequency or frequencies.
Continuously varying or changing reception conditions are also the cause of so-called multi-path interferences. This term applies to interference that arises from the superposition of signal components reaching the receiver antenna via a direct path with other signal components that reach the antenna via other indirect paths, e.g. due to reflections, and thus exhibit a phase shift. For example, such reflections arise on large buildings and the like. The overall signal arriving at the antenna thus includes multiple signal components that have reached the antenna by different paths, e.g. due to different interposed reflections, and thus have different phase shifts relative to each other. Due to such multi-path interference, the reception can vary very strongly or drastically over very small spatial distances due to the differing superposition of the various phase-shifted radio signal components. Thus, as the receiving antenna moves, the overall received signal will fluctuate or vary drastically. In this context, a second example of the above mentioned techniques comes into play, particularly with a so-called antenna diversity system. Such a system is characterized by providing plural antennas, and selecting a respective active antenna among the available antennas at any time based on the signal reception quality of the respective antenna.
A combination of the above two techniques is given, for example, by an arrangement including plural separate antennas and plural separate audio and background receivers, which are respectively coupled with their own antennas. In such an arrangement or configuration, the special requirement arises, that the various receivers must operate completely independently of one another in a first operating mode or condition, but must operate on the same frequency in a second operating mode or condition. For example, in the first operating mode, one receiver operates on the audio frequency that has been selected for the superior reception quality thereof, while the other receiver operates as the background receiver and periodically tests the reception quality available on the various alternative frequencies.
Moreover, in principle it is possible to operate plural receivers on the same frequency, and to increase the signal reception sensitivity of the overall system through the phase-correct addition of the several signals or signal components received respectively by several antennas of the overall system. This increase of the sensitivity can be achieved because the noise signals arise in an uncorrelated manner via the several antennas, while the useful signal (e.g. the audio signal that is to be received) arises in a correlated manner via the several antennas. Through appropriate phase shifting of the added signals, a directional effect of the overall antenna system can be achieved.
In typical conventional signal superimposing receivers, e.g. heterodyne receivers, a high frequency reception signal is superimposed or mixed with an oscillator signal so as to be mixed down to an intermediate frequency. In this context, it is problematic that the respective local oscillators of the various receivers must be very strongly decoupled from one another in order to avoid mutual influence therebetween. In generally known conventional methods and circuit arrangements, various different local oscillators are synchronized on a common reference frequency, and it is attempted to decouple the local oscillators from one another. However, phase noise of the oscillators as well as noise components of the phase locked loops prevent a complete or perfect synchronization of the oscillators when all of the oscillators are to operate on the same frequency. Moreover, an incomplete decoupling of the oscillators relative to one another leads to a mutual or interactive influence therebetween, which is noticeable as interfering noise in the receiver.
Alternatively, a synchronization can also be achieved in that one receiver distributes the signal of its oscillator to the other receivers. The local oscillators of the other receivers are then switched off, because the other receivers will instead use the oscillator signal provided by the first or master receiver. This alternative, however, requires relatively complicated and expensive high frequency switches, in order to ensure an adequate or sufficiently high decoupling in the switched-off condition. Since a tuning voltage from the phase locked loop of a local oscillator is used for adjusting or tuning subsequent or following filter circuits of the receiver, for this purpose the tuning voltage from the phase locked loop of the active local oscillator must also be delivered further on to the other receivers, which is complicated with respect to the necessary circuit arrangements therefor. Moreover, the tuning adjustment or balancing of the individual receivers is complicated, since an adjustment or balancing of the filter circuits is based on the control voltage of the oscillator (e.g. voltage controlled oscillator VCO). Thus, the tuning adjustment or balancing must occur in the entire or overall system, when both voltage controlled oscillators (both the internal and external master oscillator) are present. In this regard, the filter balancing is further made more difficult if the individual receivers are respectively subjected to different surrounding environmental temperatures, such that the several receivers will exhibit different temperature-dependent characteristic behaviors.